In agriculture, and particularly in the viticultural field, a well-known practice is to prearrange plant support structures consisting of rows of poles driven into the ground so as to form rows. The poles are spaced apart and are interconnected in their parts above ground by a horizontal wire, called “carrying or training wire”, that is secured on said poles at a suitable height above the ground. On the two poles that are placed at the ends of each row, called “head poles”, are also fastened at least one pair of wires parallel to each other and arranged on respective sides of each pole in the row. These wires are commonly called “containment wires” or “movable wires” because, during the different phases of cultivation of the plants, they are moved to different heights above the ground to contain and arrange the growing canopy according to a desired configuration. In winter, in fact, when the vine shoots are very short or are completely absent, the pairs of movable wires are arranged in a position closer to the ground. In the spring and summer seasons, when the plants are in full growth, each pair of movable wires is progressively moved to a position farther from the ground in the direction orthogonal to the ground, to contain the canopy between the wires of each pair, thus making it grow according to a desired configuration.
According to a well-known method, at least one of the ends of the movable wires is fastened to a head pole on the row by means of metal chains provided with a suitable coupling device or suitable clamps, which were used to fasten one of the chain links to the pole. With that fastening solution, when it is necessary to move each movable wire to another position, the wire must first be loosened by uncoupling the relative chain from the pole, then repositioning the movable wire at the desired height, and finally coupling the chain back on the pole. These operations, all of them fairly complex and strenuous, require the presence of at least two persons, one to support the chain while keeping the movable wire taut, and at least another person to move the movable wire to a new position. Thus it is evident, especially considering the fact that on a tilled land the rows are numerous and of considerable length, that such a fastening configuration of the movable wires on a head pole of a row of vines involves a considerable amount of physical labour and relatively long working times and considerable costs.
As a partial improvement of the chain-based fastening method, in the International Patent Application WO 2005/048691, is disclosed an elastic compensation device that makes it possible for a single person to vertically reposition each movable wire. According to this embodiment, at least one end of each movable wire is coupled to a first end of an elastic compensation device stably fixed to the head pole in the row.
A drawback of this solution lies in the fact that the vertical change of position of the movable wire to place it at a different height with respect to the height at which the two ends on the head poles are fastened, involves the formation of a portion of movable wire that is placed obliquely with respect to the ground in the region included between each head pole and the first pole in the row down from said head pole. This is due to the fact that the change in the height of the movable wire applied manually does not include the ends of the same wire, which remain in a fixed position on the head poles. This oblique positioning of the movable wire with respect to the ground into which the poles are driven determines the substantial impossibility of managing in the best manner the canopy growing from one or more plants placed in the area of the ground included between each head pole and the next pole down the row. It is thus evident that, although a row can theoretically accommodate plants, in particular grape vines, spaced at a distance of about 80 cm from each other, the solution of fastening the movable wires to the head poles disclosed in WO 2005/048691 implies substantially the need to place the first plant in a row at a distance greater than 1.5 meters from the head pole on which the device is fastened, in other words beyond the first pole in the row after the head pole. Thus it can be seen that, for each row which naturally includes two head poles, in order to train the plant growth in the best manner, it is necessary to give up planting at least one pair of extra vines over the number that could theoretically be accommodated by the row if they were spaced at a distance of about 80 cm from each other. In particular, in a field planted with grape vines, even if they are of relatively small size, the number of plants that must be given up planting can be a considerable disadvantage, with a consequent loss in the yield of the vineyard.
As a partial solution of the shortcomings outlined above, Italian Patent IT 1364354 discloses an adjustable and transferable tensioning system of the mobile wires, wherein at least one pair of wire tensioners are slidingly coupled to each of the two head poles. Each wire tensioning device supports two mobile wires arranged parallel to and at a short distance from each other so as to be placed on the sides of the intermediate poles in the row in order to properly train the plant growth. This system has considerable disadvantages, both in terms of the method of vertical displacement of the mobile wires and in terms of the limitations it imposes on the arrangement of a row of plants, in particular grape vines. A first one of these shortcomings lies in the fact that the vertical displacement of the mobile wires can be carried out only by simultaneously moving two mobile wires having their corresponding ends mounted on the same wire-tensioning devices. In actual practice, in fact, the simultaneous displacement of a pair of mobile wires and of the relative wire-tensioning devices is strongly hindered by the presence of the grown canopy, whose casual arrangement makes it difficult to contain it between two parallel mobile wires moved vertically at the same moment. In the system disclosed in IT 1364354, a further problem in the displacement of each pair of mobile wires is due to the high friction present between each wire-tensioning device and the head pole or the relative sliding guide on which it is mounted, which results disadvantageously in the necessity of using a particular tool to pry the device away from the pole. In addition to the friction between the contact surfaces, the tension imposed by the same wire-tensioning devices on each pair of mobile wires also hinders the vertical displacement of the latter. In addition, the fact that the operation of changing the vertical position of the mobile wires involves the simultaneous displacement of two wires that are coupled to the same wire-tensioning device and that are at least in part obstructed by the canopy, contributes, together with the problems described above, to making the operations of changing the position of the mobile wires laborious and complicated.
Still another shortcoming of the system disclosed in IT 1364354 lies in the fact that the vertical displacement of each pair of mobile wires is limited by the presence of the carrying wires in the row that extend longitudinally between all the intermediate poles in the row, starting from the two head poles, and whose position is definitively determined the moment when the row is arranged. In fact, since each pair of movable wires is secured to the same wire-tensioning device, the range of vertical movement of the latter cannot include within it the presence of any carrying wire and of the relative fastening mechanism on the head pole. Thus, in a row of grape vines, the adjustable and translatable tensioning system for the mobile wires proposed in IT 1364354, which naturally must take into account the normal heights of the plants on which the canopy to be trained grows, imposes the disadvantage of arranging each carrying wire above the highest position or below the lowest position in which it is possible to place each pair of mobile wires by sliding the relative wire-tensioning device on the head pole. This implies the necessity of using poles whose part above ground is rather high or, to limit that height, limiting the vertical distance that each pair of mobile wires may travel, with the risk that it may not be possible to effectively contain part of the canopy. Similarly to what previously described with reference to the fastening of the mobile wires using means of elastic compensation, a further disadvantage of the positioning requirements of the system disclosed in IT 1364354 with respect to the carrying wires in the row lies in the fact that the canopy growing from the vines present in the region between each head pole and the first pole on the row after the head pole cannot be easily contained by varying the position of the mobile wires, but must be disadvantageously trained on said mobile wires through laborious manual operations. In fact, according to what is disclosed in IT 1364354, at the beginning of the first phase of vegetation growth, in the region included between each head pole and the first pole in the row after the head pole, the movable wires are arranged obliquely relative to the ground, and, especially in the area proximate the head pole, they are arranged at a higher level than the fruiting canes on the plants. This arrangement allows the canopy to grow in casual configurations that position themselves outside of the area included between each pair of movable wires arranged on opposite sides of the row, thus avoiding being contained when the movable wires are being displaced vertically.
A not less significant disadvantage of the system disclosed in IT 1354354 consists of the fact that the displacement of each pair of movable wires implies the displacement of at least a further pair of wires supported on the same head poles, since said system does not allow a first pair of wires placed at a lower height to be displaced in height along the head pole and be moved past, or above, a second pair of wires placed at a higher level relative to the first pair of wires. In other words, since the training system proposed in IT 1364354 provides for mounting on a single vertical guide all the wire-tensioning devices that fasten the movable wires, when it becomes necessary to move said wires to a higher level in order to contain the grown canopy, before moving to a higher level a first wire-tensioning device located nearer to the top of the head pole it is necessary to move to a higher level a second wire-tensioning device that is located immediately below the first device until it is substantially moved up to the position occupied by the first device. Only after the second device has substantially taken the place of the first device, the latter can be lifted higher along the head pole. This double displacement, and in particular the displacement of the movable wires that in the course of the seasonal growth of the canopy have first curbed the vegetation, causes the breaking of the tendrils that anchor the vine shoots to the movable wires, thus irreparably damaging the vine. This problem is particularly sensitive for some types of grape vines that produce slender and easily breakable shoots.